Ion implantation has been utilized for the introduction of impurities into a substrate because it is a "cold" technology (which does not require the material degrading heat cycles necessary for thermal diffusion masks, and the diffusion itself), and because it is a technology whereby reproducible results can be obtained. Devices so fabricated are substantially reproducible even if the flux of the ion beam should vary significantly with time, provided only that the ion beam current can be accurately monitored to obtain the total implant dose. It has been difficult heretofore to measure beam current for very low levels of implanted impurities (doses .ltorsim. 10.sup.12 atoms/cm.sup.2) because the current may be of the order of 10.sup.-10 Amperes. For example, such low dose may be used for shifting the threshold of a field-effect transistor.
Other applications require very high impurity doses. Some accelerators commonly used for ion implantation can deliver currents above 10.sup.-2 amperes for very high levels of implanted impurities. A charge-neutral beam can be obtained by introducing electrons into the beam which keep it from dilating due to space-charge effects. However, these high current ion implantations may be accompanied by inaccurate current monitoring because of the electrons put into the beam.
Recently, in the literature of ion implantation for device fabrication, the problem of reproducibility of the dose of implanted ions has been addressed by making use of information concerning the spatial position of the beam on the substrate together with measurement of the instantaneous current to the target which allowed a final correction to be made to those areas which had received too little dose. There has also been a suggestion in the literature that monitoring of the substrate temperature could aid in the fabrication reproducibly of semiconductor devices through use of ion implantation.
The following literature articles are identified as being of background interest concerning use of X-rays emitted as a consequence of ions impinged upon a substrate for a technological purpose:
(1) W. Beezhold et al. in Applied Phys. Letters 21, 592, (1972) describes a study of ion-induced X-rays from H and He bombardment to detect charge build-up on an insulating surface.
(2) H. Kamada et al. in "6th Conference on X-ray Optics and Microanalysis, " p 541, Univ. of Tokyo Press 1972 describe spectrochemical analysis by study of low energy ion-induced X-rays.